Direct vent gas stoves and fireplaces are appliances that use a flue to vent combustion waste outside of a dwelling. Venting can occur either horizontally, through a wall, generally referred to as a rear vent, or up through the ceiling, generally referred to as a top vent. The key advantage to direct vent appliances is that they are independent of room air and use externally sourced air for combustion.
In direct venting, room air is not used for combustion. Rather, air used for combustion is drawn into the combustion chamber by use of a conduit which communicates with the outside ambient air. Typically, a direct vent pipe includes two ducts formed by an inner pipe surrounded by a larger diameter outer pipe. The outer pipe conveys outside air to the combustion chamber. After combustion, the exhaust is conveyed to the outside via the inner pipe. The two ducts are typically cylindrical and can be concentric, with the inlet air being conducted to the combustion chamber through an annulus outside the exhaust duct and the exhaust being conducted outside by way of the inner duct, co-linear (or side-by-side), or completely separate ducts.
Vent caps cover the inlet/outlet of the first and second ducts on the outside of a dwelling. Winds and drafts around the vent cap can affect the backpressure in the duct. If there is backpressure present in the exit duct, the draw of inlet air will be reduced which will decrease combustion efficiency and can lead, in poorly designed systems, to extinguishing the combustion flame.